It has been known to use silver ions for separating cis-trans isomers of an unsaturated aliphatic acid having the same number of carbon atoms (J. Chromatography, 149(1978) 417-). However, it has been difficult to separate analogous compounds which are slightly and partially different in their molecular structure by usual processes, because they have the same or almost the same number of carbon atoms and accordingly are similar to each other in physical properties such as solubility in and affinity to solvents.